Either one. I keep imagining quitting, but then pulling myself back. That thought of it'll get better in a month, or two months, etc. I'm hoping to motivate myself to march through as I once did, or to drop it and not look back. Not to be constantly conflicted.
There are a lot of lessons you can take from this project and leverage for later, on this project or other projects or for your current or future day job. If you leverage your work here for something else, and quit this project, maybe knowing your experience helped you transition will ease your mind about quitting. I'm serious about sunk cost being imaginary; you aren't losing anything you have by stopping development. The outcome was always going to be what you learned by doing it, and you have that.
Not sure, but it occurred to me that one thing you may be discovering is that interpreters (like compilers & OSes) are a different beast than apps programming. You can't necessarily use the same coding styles or keep the same clean abstractions. You are bumping into project scaling problems. We all have a point where we start producing spaghetti. Everyone. Some people have some tricks for going further, but nobody can do it forever. Ever read this post? https://www.teamten.com/lawrence/writings/norris-numbers.htm...
What you're learning about the discrepancy between what you expected to happen 6 years ago and what has actually happened since could be redirected towards a management philosophy, or even a job doing that, if you were so inclined. You mentioned you are antisocial (and we all are to varying degrees) but what's happened to this project is what happens to most projects. Most software gets ugly and less fun over time. In fact, it's probably all software.
I freaking love coding, it's addicting. But software as a whole has been making me mildly unhappy in part because it's so difficult to feel like I ever finish anything, whether I'm in a large team or a small one, or working solo. Many days I feel like I could code every waking second I have and not end up where I want to go. I have big plans and, more and more clearly every day, not enough time.
BTW, I haven't gotten rid of my constant conflict. It might not go away unless you decide to end the project. Mine is still there, and I'm still going. But I have slowed my project down and I spend less time on it. It's helping relieve some of the pressure, and helping me be more judicious about what I do. I'm starting to spend more time doing side things that are fun & interesting to me.
Maybe do a trial quit, and change your mind if you want. Walk away for three weeks and don't think about it for one second. See if you're drawn back or start to feel free.
This article is frustration embodied, and I completely empathize, having been in a very similar situation with a large and long running project that had many (thousands of) users. Motivation and enjoyment comes and goes, but I couldn't bring myself to stop doing it because of the effort it took to get there, and because people were using it and counting on it's continued development.
The author is sad and tired and can't see the big picture. It's extremely unlikely he's getting worse at programming, unless he works really hard not to learn from this -- but this article is evidence he's already learning what happens to big projects. You have to acknowledge what's going on (done) and then step back and think about how to improve the situation.
I'm not sure how to find the motivation to continue, if that's what he really wants. But at least take a solid break and then spend some non-dev time re-evaluating what's important and where you're headed. Consider whether continuing solo is a goal, or whether recruiting real partners might be what both you and the project really need. Ask what success looks like.
As for permission to quit, it's not true that giving up is a massive loss of work. All the work done is already done. Lily runs, and lessons have been learned. The code isn't going anywhere. You can walk away from this and you get to keep everything you ever put in. What you lose by quitting is future potential, not the sunk cost. So if you want to quit and the only thing holding you back is fear or sunk cost -- go ahead and quit, sunk cost is imaginary in this case.